Device and method for normalizing implant strain readings to assess bone healing

ABSTRACT

A device and method for treating a hone includes a bone plate including first and second portions joined to one another via a connecting portion, a rigidity of the connecting portion being less than rigidities of each of the first and second portions in combination with a first sensor mounted on the first portion measuring strain on the first portion and a second sensor mounted on the second portion measuring strain on the second portion.

BACKGROUND

Strain gages can be placed on orthopedic implants to track the progressof bone healing. Upon initial implantation, the implants are expected toexperience higher levels of strain which decrease during healing as thebone begins to share more of the load with the implant. Currently,however, implant strain values need to be assessed with a known loadapplied to the bone in order to evaluate bone healing.

SUMMARY OF THE INVENTION

The present invention relates to a device and method for treating a boneincludes a bone plate including first and second portions joined to oneanother via a connecting portion, a rigidity of the connecting portionbeing less than rigidities of each of the first and second portionsalong with a first sensor mounted on the first portion measuring strainon the first portion and a second sensor mounted on the second portionmeasuring strain on the second portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of a system according to a firstexemplary embodiment of the present invention;

FIG. 2 shows a perspective view of a system according to a secondexemplary embodiment of the present invention;

FIG. 3 shows a perspective view of a system according to a thirdexemplary embodiment of the present invention;

FIG. 4 shows a side view of a bone fixation element of the system ofFIG. 3 ;

FIG. 5 shows a perspective view of a system according to a fourthexemplary embodiment of the present invention;

FIG. 6 shows a top plan view of a system according to a fifth exemplaryembodiment of the present invention; and

FIG. 7 shows a top plan view of a system according to an alternateembodiment of the present invention.

DETAILED DESCRIPTION

The present invention may be further understood with reference to thefollowing description and the appended drawings, wherein like elementsare referred to with the same reference numerals. The exemplaryembodiment of the present invention relate to a system and method fortracking the progress of bone healing. In particular, the exemplaryembodiments describe systems and methods that calculate a ratio ofstrain at multiple locations along an implant and/or a bone. Anexemplary embodiment of the system may include a first sensor on asurface of the implant adapted to be positioned at a location proximatea weakened portion of the bone. Strain on the implant at this locationwill be affected by the strength or stiffness of the weakened bone andthe load placed on the bone by the patient. A second sensor may beplaced on the implant at a location in which strain measured by thesecond sensor is affected only by the load placed on the bone such thatthe measured strain is substantially unchanged by the bone healingprocess. Thus, a ratio between the strains measured by the first andsecond sensors provides information corresponding to bone healing,regardless of the load on the bone. It will be understood by those ofskill in the art that although the exemplary embodiment specificallydescribe tracking the healing progress of a leg bone, the presentinvention may be used to track the progress of healing of any loadbearing bone. It will also be understood by those of skill in the artthat although the exemplary embodiments specifically show and describetwo sensors, the present invention may include additional sensors alongdifferent areas of the bone to determine ratios corresponding to thebone healing progress of the different areas. In addition, althoughexemplary embodiments show a bone plate, the present invention may beused with any other fixation element such as, for example, screws,intramedullary devices, external fixators, spine fixation implants andprosthetics.

As shown in FIG. 1 , a system 100 according to a first exemplaryembodiment of the invention comprises an implant 102 (e.g., a boneplate) and first and second sensors 104, 106, respectively. The implant102 is configured for fixation over a target portion of a bone 108 to,for example, fix a fracture 110 or to support a weakened portion of thebone 108. The first and second sensors 104, 106 are mounted along asurface 114 of the implant 102 such that the first and second sensors104, 106 may be mechanically coupled to the bone 108. Although thesurface 114 is shown as facing away from the bone 108 when the implant102 is fixed to the bone 108 in a desired location, it will beunderstood by those of skill in the art that the sensors 104, 106 may bemounted along any surface of the implant 102. For example, the sensors104, 106 may also be mounted on a surface of the implant 102 facing thebone 108 or a surface on a side of the implant 102. The first and secondsensors 104, 106, respectively, are positioned on the implant 102 sothat, when the implant is in a desired position on the bone 108, thefirst sensor 104 is located over a site of the fracture 110 while thesecond sensor 106 is separated from the fracture 110 over a healthy(i.e., solid) portion 112 of the bone 108 to measure levels of strainand/or load on the implant 102, at these positions along the implant102. The second sensor 106 should be isolated between two screws lockedin a healthy portion 112 of the bone 108 to measure a load on the bone108.

The sensors 104, 106 in this embodiment may be passively powered MEMssensors that are used to measure strain and include an interface forwireless connection to a data collection device as would be understoodby those skilled in the art. In another embodiment, the sensors 104, 106may be powered chips that are connected to a printed circuit board(PCB). This permits strain on the implant 102 to be measured andtransmitted to the data collection device for further processing withoutphysically accessing the sensors 104, 106. It will be understood bythose of skill in the art that the strain measurements detected by thesensors 104, 106 are not required to represent actual strain values, butmay include any signal that changes based on changing strains of theirsubstrates. For example, the MEMS sensors 104, 106 may be RF devicesthat deform when a strain is placed thereon, resulting in a frequencyshift caused by a change in capacitance of the sensors 104, 106 suchthat the frequency shift corresponds to a change in strain. As would beunderstood by those skilled in the art, an external device may beemployed to wirelessly provide a signal to the sensors 104, 106. Changesin a returned signal may then be measured to determine a level of strainto which the sensor is subject. A ratio of the strain measured by thefirst sensor 104 to the strain measured by the second sensor 106 maythen be determined by a physician or other professional to track healingprogress. Alternatively, the ratio may be determined by a processingdevice that may also store the strain measurements and the determinedratios (e.g., in an internal memory or on an external storage device) sothat changes in the ratio may be reviewed to more fully understand theprogression of the healing over time.

It will be understood by those of skill in the art that when the bone108 is initially broken or fractured, strain on the implant 102 at thelocation of the fracture 110 will vary based on changing mechanicalproperties of the bone 108 during the healing process and the loadplaced on the bone 108 (e.g., the weight that the patient places on theleg) while the strain measured in the healthy portion 112 varies basedonly on the load placed on the bone 108. Thus, taking a ratio of thestrains measured by the two sensors 104, 106 normalizes the effects ofthe load on the sensors 104, 106 providing data corresponding to thestiffness of the bone 108 at the fracture site 110. The ratio of themeasurements from the first sensor 104 to the measurements from thesecond sensor 106 during the healing process should trend in adecreasing pattern over time, whereas a lack of healing would show norecognizable trend over time.

As shown in FIG. 2 , a system 200 according to a second exemplaryembodiment of the invention is substantially similar to the system 100,including an implant 202 and at least two sensors 204, 206. However,rather than both sensors 204, 206 being positioned on the implant 202,the first sensor 204 is located on a surface 214 of the implant 202 in aposition corresponding to a fracture of a bone 208, while the secondsensor 206 is placed directly on a solid portion 212 of the bone 208,outside a perimeter of the implant 202. Thus, the first sensor 204measures strain on the implant 202 at a position corresponding to thesite of the fracture 210 while the second sensor 206 measures strain onthe solid portion 212 of the bone 208. Similarly to the system 100, aratio between the strains measured by the first and second sensors 204,206 is determined and tracked to study the progress of healing in thebone 208. As indicated above, the ratio of the strain measurements fromthe first sensor 204 to the strain measurements from the second sensor206 trend in a decreasing pattern as the bone 208 heals, whereas a lackof healing will show no recognizable trend over time.

As shown in FIGS. 3-4 , a system 300 according to a third exemplaryembodiment of the invention is substantially similar to the system 200,comprising an implant 302 and at least two sensors 304, 306. Similarlyto the first sensor 204, the first sensor 304 is placed on a surface 314of the implant 302 in a location corresponding to a position of afracture 310 of a bone 308 (when the implant 302 is mounted on the bone308 in a desired position) to measure strain on the implant 302 at theposition of the fracture 310 while the second sensor 306 is placeddirectly on a solid portion 312 of the bone 308. However, rather thanbeing placed on an exterior surface of the bone 308, the second sensor306 is placed within the solid portion 312 via, for example, a bonefixation element 316 (e.g., screw).

The second sensor 306 may be attached adjacent to a proximal end 318 ofthe bone fixation element 316 such that when the bone fixation element316 is inserted into the solid portion 312 of the bone, the secondsensor 306 contacts a cortical wall of the bone 308. The second sensor306 may be printed or mounted around a portion of the bone fixationelement 316 to measure deformation of the bone 308 which is directlyrelated to strain on the bone 308. The ratio of the measurements fromthe first sensor 304 to those of the second sensor 306 may then bedetermined to track healing progress in the same manner described above.

As shown in FIG. 5 , a system 400 according to a fourth exemplaryembodiment of the invention is substantially similar to the system 100,comprising an implant 402 and first and second sensors 404, 406,respectively, both of which are mounted on the implant 402. Similarly tothe first sensor 104, the first sensor 404 is located on the implant 402in a position which, when the implant 402 is in the desired position,corresponds to the location of a fracture 410 so that the first sensor404 measures strain on the implant 402 at a position corresponding tothe site of the fracture 410. The second sensor 406 is positioned on aportion 420 of the implant 402 having greater flexibility than theportion of the implant 402 on which the first sensor 404 is mounted. Forexample, the portion 420 may be made more flexible than other portionsof the implant 402 by reducing a width (i.e., an extent of the implant402 across a bone facing surface thereof in a direction perpendicular toa longitudinal axis of the implant 402) and/or a thickness of theportion 420 (i.e., a distance between the bone facing surface and asurface thereof which faces away from the bone) as compared to remainingportions of the implant 402. In a preferred embodiment, the flexibleportion 420 is adjacent to an end 422 of the implant 402 so that thesecond sensor 406 is separated from the fracture 410 by a distance greatenough to ensure that the underlying portion 412 of the bone 408 issolid.

The second sensor 406 on the flexible portion 420 of the implant 402 isfixed to the solid portion 412 of the bone 408 via, for example, lockingscrews inserted in holes 424 on opposing sides thereof. The secondsensor 406 measures strain on a portion of the implant 402 correspondingto the solid portion 412 of the bone 408 so that measurements from thesecond sensor 406 may be used to normalize measurements from the firstsensor. Similarly to the placement of a sensor directly in or on a bone,as described in conjunction with systems 200 and 300, placing the secondsensor 406 on a more flexible portion 420 of the implant 402 between twolocked screws permits a more accurate measurement of the strain on theunderlying solid portion 412 of the bone 408, as compared to the resultsfrom placing the second sensor 406 on a stiffer portion of the implant402. The ratio of the measurements from the first sensor 404 to themeasurements from the second sensor 406 during the healing processshould trend in a pattern indicating an increasing stiffness of the bone408 over time, whereas a lack of healing should show no recognizabletrend over time.

As shown in FIG. 6 , a system 500 according to another exemplaryembodiment of the present invention may be substantially similar to thesystem 100, comprising a bone plate 502 and first and second sensors504, 506, respectively. The first and second sensors 504, 506 aremounted along a surface 514 of the bone plate 502 such that the firstand second sensors 504, 506 may be mechanically coupled to a bone viathe bone plate 502. The first and second sensors 504, 506 are positionedon the bone plate 502 so that when the bone plate 502 is in a desiredposition along the bone, the first sensor 504 is located over a site ofa fracture of the bone while the second sensor 506 is separated from thefracture (preferably over a healthy (e.g., solid) portion of bone) asdescribed above in regard to the system 100. The first and secondsensors 504, 506 measure a level of strain on the bone plate 502 atthese positions of the bone plate 502 and a ratio of the strainsmeasured by the first and second sensors 504, 506 indicates aprogression of healing of the bone over time.

In contrast to the system 100, the bone plate 502 includes a firstportion 522 and a second portion 524 connected to one another via aconnecting portion 520 with the first sensor 504 mounted to the firstportion 522 and the second sensor 506 is mounted to the second portion524 on a side of the connecting portion 520 opposite the first portion522. Thus, when the first portion 522 is positioned such that the firstsensor 504 overlies a fracture site while the second portion 524 ispositioned such that the second sensor 506 extends over a healthyportion of bone, the connecting portion 520 extends between the firstand second sensors 504, 506. The connecting portion 520 is designed toreduce strain transmitted between the first and second portions 522,524. Specifically, the connecting portion 520 is less rigid than thefirst and second portions to reduce a torsion strain applied to thesecond sensor 524. For example, a width of the connecting portion 520may be reduced relative to the widths of the first and second portions522, 524 to render the connecting portion 520 more flexible than thefirst and second portions 522, 524. This reduces the rigidity of thebone plate 502 across the surface 514 in a direction substantiallyperpendicular to a longitudinal axis of the bone plate 502, moresubstantially mechanically separating the second sensor 506 from strainsassociated with the fracture site (i.e., reducing torsion strainsapplied across the fracture site and transmitted to the second sensor506) so that the second sensor 506 more accurately measures levels ofstrain associated only with the healthy portion of bone. It will beunderstood by those of skill in the art that the first sensor 504, whichis located over the fracture site, detects bending strain which aresignificantly greater than any torsion strains detected thereby so thatthese torsion strains have an insubstantial impact on the total strainmeasured by the first sensor 504.

In an another example, the connecting portion 520 may be formed as afrangible link designed to fail when a torsion force applied theretoexceeds a predetermined threshold level. Thus, before the frangible linkis broken, the second sensor 506 is subject to torsion strains limitedby the threshold level and, after the frangible link has been severed,the first and second portions 522, 524 of the bone plate 502 arecompletely separated and isolated from one another eliminating theimpact on the second sensor 506 of any torsion strains arising over thefracture site. Those skilled in the art will understand that thethreshold level at which the connecting portion 520 fails is preferablyset so that, when the frangible link is not broken, the maximum level oftorsion strain transmitted from the first portion 522 to the secondsensor 506 are insignificant as compared to the strain levels expectedto be seen in the healthy bone. In another alternate embodiment, asshown in FIG. 7 , a system 500′ comprises two separate bone plates 522′,524′ on which first and second sensors 504′, 506′ are mounted,respectively. The first and second bone plates 522′, 524′ are notconnected to one another allowing the first plate 522′ to be attachedover a fracture site while the second plate 524′ is separated therefromand attached over a healthy portion of bone to completely eliminate thetransmission of torsion from the fracture site to the second sensor506′.

It will be understood by those of skill in the art that other mechanismsmay be employed for normalizing measurements of strain on a portion ofan implant which, when mounted on a bone in a target location,corresponds to a position of a fracture or other weakened portion ofthat bone. For example, the patient may be provided with load sensors onwhich to push or stand with the affected limb such that a loadmeasurement may be taken simultaneously with a strain measurement of thesensor on the implant. Alternatively, the patient may be provided with asensor (e.g., placed in the sole of a shoe) to measure the load placedon the affected leg, if the affected bone is the femur or tibia.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the structure and themethodology of the present invention, without departing from the spiritor the scope of the invention. Thus, it is intended that the presentinvention cover the modifications and variations of this inventionprovided that they come within the scope of the appended claims andtheir equivalents.

What is claimed is:
 1. A method, comprising: coupling a first portion ofa bone fixation device to a bone with a first sensor of the bonefixation device positioned adjacent to a fracture site; coupling asecond portion of the bone fixation device to a heathy portion of thebone; measuring a strain on the first portion of the bone fixationdevice including the first sensor; measuring a strain on the secondportion of the bone fixation device including a second sensor; anddetermining a ratio of the strain measured by the first sensor to thestrain measured by the second sensor to normalize an effect of loads onthe bone and determine a level of healing of the hone.
 2. The method ofclaim 1, wherein the second sensor is coupled to the second portion ofthe bone fixation device, and further comprising coupling the secondportion of the bone fixation device to the healthy portion of the bonevia first and second bone screws positioned on opposite sides of thesecond sensor.
 3. The method of claim 1, wherein the second sensor isdirectly coupled to the healthy portion of the bone.
 4. The method ofclaim 2, wherein the first and second portions of the bone fixationdevice are connected to one another via a connecting portion of the bonefixation device which has a rigidity less than each of the first andsecond portions of the bone fixation device to reduce a level of straintransmitted between the first and second portions of the bone fixationdevice through the connection portion of the bone fixation device. 5.The method of claim 4, wherein the bone fixation device has a lengthalong a longitudinal axis thereof and a width transverse thereto, theconnecting portion having a width smaller than widths of the first andsecond portions of the bone fixation device.
 6. The method of claim 4,wherein the hone fixation device is a bone plate.
 7. The method of claim4, wherein the hone fixation device is an intramedullary device.
 8. Themethod of claim 4, wherein the connecting portion is formed as afrangible link designed to fail when a torsion to which the frangiblelink is subjected exceeds a predetermined level.
 9. The method of claim1, wherein each of the first and second sensors is one of a MEM sensor,a strain gauge and a powered chip connected to a printed circuit board.10. The method of claim 1, wherein the first and second sensors areconfigured to permit data collected thereby to be read wirelessly via adevice external to a body within which the first and second sensors havebeen implanted.
 11. The method of claim 3, wherein the second sensor isincluded on a bone screw inserted into the healthy portion of the hone.12. The method of claim 1, wherein the first sensor includes a passivelypowered chip.
 13. The method of claim 1, wherein the first and secondsensors wirelessly provide data to an external data gathering unit. 14.The method of claim 1, wherein the ratio of the strain measured by thefirst sensor to the strain measured by the second sensor is determinedby a processing unit.
 15. The method of claim 2, wherein the firstportion of the bone fixation device is not connected to the secondportion of the hone fixation device.
 16. The method of claim 3, whereinthe second sensor is positioned on an exterior surface of the healthyportion of the bone.
 17. The method of claim 1, wherein the bonefixation device is a bone plate, further comprising coupling the boneplate to the bone via a plurality of screws inserted through a pluralityof variable angle holes on an exterior surface of the bone plate, thevariable angle holes running through the bone plate from the exteriorsurface of the bone plate to an interior surface of the bone plate, theinterior surface contacting the bone.